The present invention relates to an axial fan, particularly for use in motor vehicles for agricultural use, intended to provide particularly silent operation.
More specifically, the invention relates to an axial fan comprising
a central hub,
a plurality of blades which extend from the hub to the periphery and which are curved forwardly in the direction of rotation of the fan and
an outer ring coaxial with the hub, to which the peripheral ends of the blades are connected.
An axial fan of the said type is described, for example, in the U.S. Pat. No. 4,358,245. The said fan according to the prior art is constructed in one single piece from moulded plastics material and has blades which have a very marked forwards curvature. In the axial projection of the said fan the median line through each blade always has a positive angle of curvature, increasing steadily from the hub to the outer ring.
In the said U.S. patent it is furthermore stipulated that the angular extension of the median line of each blade must be greater than half of the distance between two adjacent blades.
It follows that, in the case of the fan according to U.S. Pat. No. 4,358,245, the length of the blades is fairly considerable and this has repercussions on the amount of material needed to produce the blades and therefore the weight of the fan. Furthermore, the considerable length of the blades results in a lessening of the vibration frequencies of the said blades, and this may lead to a certain instability and a reduction in performance and in the silent running of the fan.
To overcome these drawbacks, it has been proposed to produce axial fans in which, proceeding from the hub towards the outer ring, each blade is firstly curved rearwardly in respect of the direction of rotation of the fan and therefore in its peripheral portion it is forwardly incurvate. Solutions of this type, which make it possible already to achieve a certain reduction in the length of the blades, are described, for example, in U.S. Pat. Nos. 4,569,631 and 4,684,324.
The fans disclosed by the said documents are intended to be produced in one single piece in moulded plastics material and are intended to be used in conjunction with the radiator of a motor vehicle.